NOx, Hg, and SO2 removal using alkali hydroxide

ABSTRACT

A process and apparatus for removing SO 2 , NO, and NO 2  from a gas stream having the steps of oxidizing a portion of the NO in the flue gas stream to NO 2 , scrubbing the SO 2 , NO, and NO 2  with an alkali scrubbing solution, and removing any alkali aerosols generated by the scrubbing in a wet electrostatic precipitator. The process can also remove Hg by oxidizing it to oxidized mercury and removing it in the scrubbing solution and wet electrostatic precipitator. Alkali sulfates, which are valuable fertilizers, can be withdrawn from the rubbing solution.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of U.S. Ser. No. 09/683,267, filed Dec.6, 2001, now U.S. Pat. No. 6,936,231.

BACKGROUND OF INVENTION

a. Field of the Invention

This invention relates to methods and apparatuses for removing NOx, Hg,and SO₂ from a gas stream.

b. Description of the Related Art

Fossil fuels are burned in many industrial processes. Electric powerproducers, for example, burn large quantities of coal, oil, and naturalgas. Sulfur dioxide (“SO₂”), nitrogen oxide (“NO”), and nitrogen dioxide(“NO₂”) are some of the unwanted byproducts of burning any type offossil fuel. Mercury (“Hg”) is often also found in fossil fuels. Thesebyproducts are known to have serious negative health effects on people,animals, and plants, and a great deal of research has been done to finda way to economically remove them from flue gas streams before theyenter the atmosphere.

SO₂ is often removed from gas streams (“desulfurization”) by scrubbingthe gas with an aqueous ammonium sulfate solution containing ammonia.Examples of this process are disclosed in U.S. Pat. Nos. 4,690,807,5,362,458, 6,277,343, and 6,221,325, which are not admitted to be priorart by their mention in this Background section. The absorbed sulfurcompounds react with ammonia to form ammonium sulfite and ammoniumbisulfite, which are then oxidized to form ammonium sulfate and ammoniumbisulfate. The ammonium bisulfate is further ammoniated to form ammoniumsulfate. The process does not remove NO or NO₂, however, which must thenbe dealt with using a different process.

NO and NO₂ (together known as “NOx”) can be removed from a gas stream bycontacting the gas stream with either ClO₂ or O₃ to convert NO into NO₂,and then scrubbing with an aqueous solution of a sulfur-containingreducing compound of alkali metals or ammonia, and a catalytic compound.Such a process is disclosed in U.S. Pat. No. 4,029,739, by Senjo et al.,which is not admitted to be prior art by its mention in this Backgroundsection. This process, however, does not remove SO₂, and requires theaddition of chlorine or ozone into the system by some other means.

Some processes exist that remove both NOx and SO₂. In one such processdisclosed in U.S. Pat. No. 4,035,470, by Senjo et al., which is notadmitted to being prior art by its mention in this Background section,NO is oxidized to NO₂ by contacting the gas with either ClO₂ or O₃ asabove. Then the SO₂ is scrubbed with a sulfite and an oxidationretardant that suppresses oxidation of the sulfite to sulfate. Iron orcopper compounds can also be added to depress oxidation. Optionally,ammonium hydroxide can be added to make sulfite and to react with CO₂ inthe gas stream to make carbonate. Like in U.S. Pat. No. 4,029,739mentioned above, this process requires the addition of either chlorineor ozone, and further requires a consumable sulfite oxidation retardant.The referenced patent did not mention whether the byproducts includedany valuable material like ammonium sulfate. However, both U.S. Pat.Nos. 4,029,739 and 4,035,470 require the addition of chlorine to a gasstream that is eventually released to the atmosphere, creating a serioussafety concern.

Yet another process for removing NOx and SO₂ from a gas stream isdisclosed in U.S. Pat. No. 4,971,777, by Firnhaber et al., which is notadmitted to be prior art by its inclusion in this Background section. Inthis process, NO is oxidized to NO₂ by the addition of organic compoundswhich decompose into radicals at high temperatures. Then an aqueousammonia solution in which the pH is adjusted to be below 5.0 absorbs theNOx and SO₂. Firnhaber teaches the importance of holding the scrubbingsolution to a low pH, since higher pH levels produce aerosols of theammonia salts that he says is an environmental burden to be thwarted.Ammonia aerosols are formed by gas phase reactions of ammonia vapor inthe scrubber and create a blue haze or white vapor that emanates fromthe stack. This is also called “ammonia slip.” Free ammonia in theatmosphere would be a serious health and environmental hazard. Firnhaberdismisses the possibility of aerosol removal means due to prohibitiveinvestment costs and high pressure loss, for instance.

The typical method of removing mercury is to add activated carbon to thegas stream in order to absorb the mercury, and then mercury-containingactivated carbon is collected in a bag house. This has the disadvantageof requiring an expensive particulate additive just to absorb onematerial, and a requiring bag house to collect the particles that wereadded.

What is needed, therefore, is a process that removes SO₂, Hg, NO, andNO₂ from a gas stream that does not require addition to the gas streamof a catalyst, chlorine, ozone, or activated carbon, can occur atrelatively high pH, and does not result in ammonia slip.

SUMMARY OF INVENTION

The present invention is directed to a process and apparatus thatremoves SO₂, Hg, NO, and NO₂ from a gas stream that does not require theaddition of a catalyst, chlorine, ozone, or activated carbon, occurs ata relatively high pH, and does not result in ammonia slip. A processthat satisfies these needs comprises the steps of oxidizing NO to NO₂and Hg to oxidized mercury, scrubbing SO₂, NO, and NO₂ from the flue gasstream with an alkali scrubbing solution having a pH greater than six,and removing any alkali aerosols generated by the scrubbing steps andoxidized mercury with an aerosol removal means. A portion of theoxidized mercury may be removed from the gas stream with the scrubbingsolution. These and other features, aspects, and advantages of thepresent invention will become better understood with reference to thefollowing description, drawings, and claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a process flow chart showing the process of the presentinvention.

FIG. 2 is a cut-away view of an apparatus according to the presentinvention.

FIG. 3 is a chart showing final NO₂ concentration from various alkalisas a function of pH.

FIG. 4 is a chart showing final SO₂ concentration from various alkalisas a function of pH.

DETAILED DESCRIPTION

The present invention is a process and apparatus for removing SO₂, Hg,NO, and NO₂ from a gas stream, especially from the flue gas stream of afossil fuel boiler. In practice, flue gas from the combustion of fossilfuel nearly always contains more NO than NO₂, and often contains Hg,which can also be removed from the gas stream by this invention.

The inventors are familiar with methods and apparatuses for removing SO₂and NOx from gas streams. U.S. Pat. Nos. 5,871,703, and 6,117,403 teachthe use of an electrical discharge apparatus to oxidize SO₂ and NOx toform sulfuric and nitric acids respectively, collecting the acids in awet electrostatic precipitator (“WESP”) to form an effluent, andprocessing the effluent to make industrial grade acids that can be sold.It also teaches converting NO to NO₂. The inventors on these two patentsare Alix, Neister, and McLarnon, two of whom are inventors of thepresent invention. U.S. Pat. No. 6,132,692 teaches the use of adielectric barrier discharge (“DBD”) reactor to form the same acids,collecting them in a WESP, and draining them from the WESP to removethem from a gas stream. The inventors on this patent are Alix, Neister,McLarnon, and Boyle, two of whom are inventors of the present invention.The above three patents were owned by the owner of the present inventionas of the filing date of this specification. They are herebyincorporated by reference as if completely rewritten herein.

The present invention comprises a three-step process as shown in FIG. 1.A gas stream comprising SO₂, NO, NO₂, and perhaps Hg, are present priorto the first step 60. The first step 60 is oxidizing at least a portionof the NO in the flue gas to NO₂ with an oxidizing means. The meansselected should be able to oxidize greater than about two percent of theNO to NO₂, and is preferably in the region of about ninety percent.

The oxidizing step should be adjusted so that the resulting mole ratioof SO₂ to NO₂ after the oxidizing step should be at least 2 to 1. Theratio is preferably four to one, but can be greater. The oxidizing means60 can be any means known in the art, including but not limited to usingan electrical discharge reactor, and injecting ClO₂, O₃ or certainorganic compounds. For example, U.S. Pat. Nos. 4,029,739 and 4,035,470teach converting NO to NO₂ by the addition of ClO₂ or O₃ into the gasstream. U.S. Pat. No. 4,971,777 teaches the addition of certain organiccompounds that decompose into radicals at high temperatures.

Examples of suitable electrical discharge reactors include corona,pulsed corona, e-beam, and DBD. DBD is synonymously referred to assilent discharge and non-thermal plasma discharge. It is not the same ascorona discharge or pulsed corona discharge. The preferred embodimentuses a DBD reactor, such as that disclosed in U.S. Pat. No. 6,132,692,by Alix, et al. In practice, the operator of the process will adjust thepower input to the reactor to attain the desired oxidation results as afunction of the cost of power input to the reactor, desired scrubbingresults, and other factors. Laboratory testing has shown that oxidationof at least 90% of the NO and Hg is readily attainable with the presentinvention.

As taught in U.S. Pat. No. 6,132,692, a DBD reactor will oxidize atleast a portion of the NO and NO₂ in a gas stream to nitric acid, and atleast a portion of the SO₂ in a gas stream to sulfuric acid. These acidsare dealt with in the next step of the process.

If oxidizing means other than an electrical discharge reactor is used,Hg may or may not be oxidized to oxidized mercury or HgO. As used inthis specification, the term “oxidized mercury” is intended to includeany or all of the forms of oxidized mercury that are known in the art,including without limitation, HgO and Hg⁺⁺. On the other hand, it ispossible, and perhaps desirable, that some of the NO and NO₂ becomesfurther oxidized to form HNO₃ regardless of the means used. The reasonwhy this may be desirable will be made clear later in thisspecification.

Another way to carry out the oxidizing step 60 is adding an alkene suchas ethene or propene to the flue gas followed by oxidizing NO to NO₂with an oxidizing means, such as the electrical discharge reactor. Thiswould have the advantage of reducing the power input requirement of theelectrical discharge reactor to get the same amount of NO to NO₂oxidation. The alkene can be added in about a 0.3:1 molar ratio ofpropene to NO, or 0.5 molar ratio of ethene to NO. The likely chemicalreaction mechanisms for the conversion of NO to NO₂ in the case whereethene is the alkene selected for use with an electrical dischargereactor are as follows:C₂H₄+OH→HOCH₂CH₂  (1)HOCH₂CH₂+O₂→HOC₂H₄OO  (2)NO+HOC₂H₄OO→NO₂+HOC₂H₄O  (3)HOC₂H₄O+O₂→HOCH₂CHO+HO₂  (4)NO+HO₂→NO₂+OH  (5)

In any event, the output gas stream comprises SO₂, less NO, more NO₂,perhaps HNO₃, perhaps H₂SO₄, and perhaps oxidized mercury, as shown inFIG. 1.

The second step 62 is scrubbing at least a portion of the SO₂, NO, andNO₂ present in the gas stream with an aqueous alkali or ammoniumscrubbing solution. Some of the oxidized mercury may also be scrubbed inthis step 62. For the purposes of this specification, ammonium shall beconsidered an alkali. The term “scrubbing” typically means “absorbing”to people having skill in the art, meaning that SO₂, NO, and NO₂ isabsorbed by the aqueous solution. However, it is intended that the term“scrubbing” as used in this specification also includes adding anhydrousammonia to the scrubbing solution to initiate the reactions leading tothe oxidation of SO₂ and reduction of NO₂.

The solution preferably comprises at least one alkali, alkali sulfite,alkali sulfate, and water. The alkali can be any one that is capable ofmaintaining the scrubbing solution at the desired pH. Preferably, thealkalis are one or more of ammonium, sodium, and potassium. The solutionpreferably has a pH greater than six, which is much higher than thattaught by Firnhaber. Firnhaber teaches that the pH must be kept to lessthan five, and is preferably 4.5, to prevent the formation of aerosols.However, the present invention is not concerned with avoiding theformation of aerosols because it includes an aerosol removal means 64,described later in this specification.

Maintaining a relatively high pH has several benefits. It increases thespeed of absorption of SO₂. It increases the ratio of sulfite availablein solution compared to bisulfite, which facilitates the oxidation ofSO₂ and reduction of NO₂. The ratio of sulfite to bisulfite is highlydependent on pH level. From these benefits, it follows that theabsorption vessel, shown as item 44 in FIG. 2, can be substantiallysmaller than that used to scrub the same amount of SO₂ in a conventionallimestone scrubber which is the most typical SO₂ scrubber in use today.In addition, the amount of scrubbing liquid required and the liquid togas ratio can be reduced. It is estimated that the size of theabsorption vessel 44 can be reduced by half, and the liquid to gas ratiocan be reduced by 75%. Because the cost of the absorption vessel andliquid circulating equipment represent a large fraction of the totalcost of a scrubber, the ability to substantially reduce the size of thevessel and associated pumps and piping is a major advantage of thepresent invention over the prior art.

Although FIG. 1 shows alkali hydroxide being added at this step,anhydrous ammonia in the form of ammonium hydroxide can be addedinstead. The alkali hydroxide reacts with the gas stream output from theoxidizing step, forming alkali sulfite and alkali bisulfite. The likelychemical reactions in this step, in the case of ammonia, potassiumhydroxide, and sodium hydroxide, are as follows:NH₃+H₂O+SO₂→NH₄HSO₃  (6)NH₄HSO₃+NH₃→(NH₄)₂SO₃  (7)2NH₄OH+SO₂→(NH₄)₂SO₃+H₂O  (8)2KOH+SO₂→K₂SO₃+H₂O  (9)2NaOH+SO₂→Na₂SO₃+H₂O  (10)

An oxidation inhibitor can be added at this step to inhibit theoxidation of sulfite to sulfate before the sulfite can perform its NO₂reduction function. Examples of oxidation inhibitors includethiosulfate, thiourea, sulfide, and emulsified sulfur.

The alkali bisulfite and alkali sulfite react with the NO and NO₂ toform their alkali bisulfate and alkali sulfate. Ammonium sulfate andpotassium sulfate, for example, are well known as valuable agriculturalfertilizers. The likely reactions that take place in this step, in thecase where potassium is the alkali, are as follows:2NO₂+4K₂SO₃→4K₂SO₄+N₂  (11)NO+NO₂+3K₂SO₃→3K₂SO₄+N₂  (12)

There are corresponding reactions for the case when ammonium and/orsodium are used as the alkali.

Most of the HNO₃ that may have been formed by further oxidation of NOand NO₂, and/or created by a DBD reactor, will react with the alkali andform its alkali nitrate. Potassium nitrate and ammonium nitrate are alsoknown to be valuable agricultural fertilizers, and are formed accordingto the following formulae:HNO₃+NH₃→NH₄NO₃  (13)HNO₃+KOH→KNO₃+H₂O  (14)

If another alkali is used, it will react with the nitric acid to formalkali nitrate. In a similar way, most of the sulfuric acid created bythe DBD reactor will react with the solution and form alkali bisulfate,which can be further reacted to form an alkali sulfate. As one can seefrom the above equations, the process removes SO₂, NO, and NO₂ from thegas stream, and produces alkali nitrate, alkali sulfate, and nitrogen.Over time, the alkali sulfate and alkali nitrate will concentrate in theaqueous alkali scrubbing solution and precipitate out of solution. Thesolid precipitate can then be removed from the scrubber and processedfor use as fertilizer, or land filled.

The gas stream after the scrubbing step comprises nitrogen and water.Since the pH of the scrubbing solution is higher than about five, theoutput from the scrubbing step will likely contain aerosols and ammoniavapor if ammonia was used. If not collected in the scrubbing solution,the gas stream will also contain oxidized mercury.

The third step 64 is removing at least a portion of the aerosols and theremainder of the oxidized mercury, if present, from the gas stream. Awet electrostatic precipitator (“WESP”) and/or mist eliminator may beused as the aerosol removal means. A WESP is effective at collectingalkali aerosols, oxidized mercury, and any other aerosols or particlesthat may be present in the gas stream.

As a result of this three-step process, SO₂, NO, NO₂, and Hg are removedfrom a gas stream to provide alkali sulfate and alkali nitrate. Theoutput of the aerosol removal means comprises N₂ as a result of theprocess of the present invention.

An apparatus according to the present invention is shown in FIG. 2. Agas stream 14 comprising SO₂, NO, NO₂, and perhaps Hg enters theapparatus assisted by a forced draft fan 12. The gas then enters a meansfor oxidizing 10 at least a portion of the NO in the gas stream to NO₂.The oxidation means 10 performs the oxidizing step 60 shown in FIG. 1,which is more fully described above. In the preferred embodiment, atleast one DBD reactor is used as an oxidizing means, and can be providedin modules 16 to facilitate manufacture and installation. At least onepower supply and controller is required to operate a DBD reactor, whichare selected by those having skill in the art, but are not shown in thedrawings.

After the oxidation means 10, the gas stream 18 comprises less SO₂, lessNO, more NO₂, perhaps HNO₃, perhaps H₂SO₄ and perhaps oxidized mercury.The gas stream temperature at this point is about 175° C. (350° F.). Thegas stream then enters a scrubbing vessel 44 into a region 19 over anaqueous alkali sulfate solution 22. Preferably, the aqueous alkalisulfate solution comprises the alkali, alkali sulfite, alkali sulfate,and water. Some of the water in the alkali sulfate solution 22evaporates due to the heat of the incoming gas stream 18, thusconcentrating the solution. A portion of the solution 22 is pumped by acirculation pump 50 to a filtration/granulation apparatus 54, which isshown in FIG. 2 as a black box. The apparatus 54 actually comprises manyparts, and is readily designed by persons having ordinary skill in theart. The output of the filtration/granulation apparatus 54 is a solidalkali sulfate/nitrate product 56, which can be sold as an agriculturalfertilizer, depending on the alkali used.

Air 17 is introduced into the alkali sulfate solution 22 for oxidizingalkali sulfite into alkali sulfate. Alkali sulfate solution 22 is pumpedwith a circulation pump 50 also to a set of lower spray nozzles 24 thatserve to cool and saturate the gas stream 18 with the aqueous solution.Some of the solution is also pumped by circulation pump 50 to the WESPspray header 42 to wet the WESP 40 electrode surfaces.

Another circulation loop is provided wherein aqueous alkali sulfite andsulfate in a vessel 48 is pumped with a circulation pump 52 to a set ofupper spray nozzles 34. The liquid then falls to a dual flow tray 30. Aseparator tray 26 allows some of the liquid to fall into the alkalisulfate solution 22, and the remainder is piped to the vessel 48. Makeupwater 20 is added directly to the vessel 48. Makeup alkali 32 is addedto the upper spray nozzles 34. These two circulation loops,independently or together, perform the scrubbing step 62 of FIG. 1,which is described in detail above.

Following the scrubbing loops, a WESP 40 is provided to remove anyoxidized mercury leaving the scrubbing loop and alkali aerosols that mayhave formed earlier in the process. The WESP 40 is preferably ashell-and-tube type of WESP, but can be a plate type, or any WESP suchas is known by those having skill in the art. The WESP 40 is wettedusing a set of sprays 42 fed from the scrubbing solution 22 andcirculation pump 50. A bubble cap tray 38 can be provided below the WESP40. A portion of the liquid in the bubble cap try 38 may be diverteddirectly to the scrubbing solution section 22 before failing through thescrubbing loops. The WESP 40 is an example of the aerosol removal means64 described in FIG. 1, which can also be a mist eliminator alone or acombination of mist eliminators and WESPs. The gas stream exiting theWESP 40 has considerably less NOx and SO₂ than that which entered theprocess and apparatus, and has an increased amount of the reactionproducts, which are nitrogen and water.

The following laboratory-scale examples of the process demonstrate theefficacy of the present invention:

EXAMPLE 1

An absorption test was done for the scrubbing step of the process of thepresent invention, with a solution that was 1% w/w SO₃ ²− (“sulfite”),6% w/w SO₄ ²− (“sulfate”), and 2.5% S₂O₃ ²− (“thiosulfate”) in a packedcolumn that was 46 cm (18 inches) high and 3.8 cm (1.5 inches) indiameter. The column was packed with 6.35 mm (0.25 inch) glass RASCHIGrings. The simulated flue gas at the inlet of the column contained 13%v/v moisture, 6% v/v O₂ and the simulated flue gas pollutants listed inTable 1. There was continuous addition of NH₃ and (NH₄)₂S₂O₃ to maintaina pH of 6.8 and a thiosulfate concentration of 2.5% w/w. The residencetime in the column was 1.8 sec with an L/G ratio of 25 gpm/kacfm.

Table 1 shows the concentrations of NO, NO₂, and SO₂ at the inlet andoutlet of the test system.

[t1]

TABLE 1 Scrubbing Step Alone System Inlet System Outlet NO (ppmv) 20 4NO₂ (ppmv) 250 36 SO₂ (ppmv) 1370 2

EXAMPLE 2

An absorption test was done to determine steady state operatingconditions for a system with inlet flue gas conditions of 7 ppmv NO, 248ppmv NO₂, 1485 ppmv SO₂, 6% v/v O₂ and balance N₂. NaOH and NaS₂O₃ wereadded to the solution to maintain a pH of 8.0 and thiosulfateconcentration of 1%. The concentrations of sulfite and sulfate in thesystem were allowed to build to steady state. The NO_(x) removal ratewas 77% at concentrations of SO₃ ²⁻, SO₄ ²⁻, and S₂O₃ ²− of 1.25% w/w,9.5% w/w, and 1.0% w/w respectively.

EXAMPLE 3

Tests were conducted in a laboratory test facility for the NO oxidizing,scrubbing, and aerosol removal steps of the process of the presentinvention. The equipment consisted of a simulated flue gas deliverysystem, a coaxial cylinder DBD reactor, a packed column scrubber and atubular WESP. The following is an example of data obtained in the labtest facility.

Simulated flue gas was delivered to the DBD reactor at a flow rate of0.4 m³/minute (14 scfm), a temperature of 143° C. (290° F.) and with thefollowing composition: 6.2% v/v O₂, 14.2% v/v CO₂, 8.2% v/v H₂O, 20 ppmvCO, 250 ppmv C₂H₄, 1740 ppmv SO₂, and 259 ppmv NO_(x). Gas velocitythrough the discharge reactor was 15.2 m/s (50 ft/s) with a dischargepower level of 140 watts. Gas from the discharge reactor entered a 10.2cm (4 inch) ID packed column scrubber, packed with 1.3 cm (0.5 inch)INTALOX saddles to a depth of 1.2 m (4 ft.). Liquid was introduced atthe top of the scrubber at a flow rate of 1.25 liters/minute (0.33 gpm)(L/G=20 gpm/kacfm). Aqueous ammonia was added to and effluent liquidremoved from the recirculating scrubber solution to maintain a constanttotal liquid volume and solution pH at 6.6. Gas from the packed bedscrubber was treated in a 10.2 cm (4 inch) ID wetted wall electrostaticprecipitator with a gas residence time of 0.7 seconds.

Table 2 below shows the concentrations of NO, NO₂ and SO₂ at the inletto the system, the outlet of the barrier discharge reactor and at theoutlet of the system.

[t2]

TABLE 2 Three Step Process System Inlet Discharge Reactor Outlet SystemOutlet NO (ppmv) 254 45 32 NO₂ (ppmv) 5 109 9 SO₂ (ppmv) 1740 1598 1

FIG. 3 is a chart showing actual experimental data of final NO₂concentrations in a process according to the present invention as afunction of pH. The results using sodium, ammonium, and potassium areshown as different data sets. The initial conditions were a temperatureof 54° C. (130° F.), 0.5% SO₃ ²− concentration, and initial SO₂concentration of 1500 ppm. FIG. 4 is a chart showing actual experimentaldata of final SO₂ concentrations in a process according to the presentinvention as a function of pH. The results using sodium, ammonium, andpotassium are shown as different data sets. The initial conditions werea temperature of 54° C. (130° F.), 0.5% SO₃ ²− concentration, andinitial NO₂ concentration of 250 ppm.

The three-step process and apparatus described herein was designedspecifically to treat flue gas from a coal fired power plant. However,it can be appreciated that the invention is capable of operating on anygas stream in which NOx and SO₂ are present, including but not limitedto gas and oil-fired boilers and various chemical manufacturingprocesses. The NOx and SO₂ concentrations and operating conditions willbe different in each situation. Therefore, it is understood that anoperator or system designer will be motivated to modify the scrubbingstep 62 to possibly eliminate the need for either one or both theoxidizing step 60 or the aerosol removal step 64, or combine the threeelements somehow so that fewer than three steps are needed.

It will be apparent to those skilled in the art that various changes andmodifications can be made without departing from the spirit of thepresent invention. Accordingly, it is intended to encompass within theappended claims all such changes and modifications that fall within thescope of the present invention.

1. A process for removing SO₂, NO, and NO₂ from a gas stream comprisingthe steps of a. oxidizing at least a portion of NO in a gas stream toNO₂ with an electrical discharge reactor, said oxidizing step furthercomprising adding an alkene to the gas stream upstream of the electricaldischarge reactor, followed by b. scrubbing at least a portion of SO₂,NO, and NO₂ from the gas stream with a scrubbing solution comprising analkali hydroxide in an amount sufficient to maintain a pH greater than5, and c. removing at least a portion of any alkali aerosols generatedfrom the scrubbing step from the gas stream with an aerosol removalmeans.
 2. The process of claim 1, wherein said alkene is at least onetaken from the group consisting of ethene and propene.
 3. The process ofclaim 1, wherein said electrical discharge reactor is a dielectricbarrier discharge reactor.
 4. The process of claim 3, further comprisingthe step of oxidizing at least a portion of the NO to HNO₃ with saiddielectric barrier discharge reactor.
 5. A process for removing SO₂, NO,NO₂, and Hg from a gas stream comprising the steps of a. oxidizing atleast a portion of NO in a gas stream to NO₂, and at least a portion ofthe Hg in a gas stream to oxidized mercury, with an electrical dischargereactor, said oxidizing step further comprising adding an alkene to thegas stream upstream from the electrical discharge reactor, followed byb. scrubbing at least a portion of SO₂, NO, and NO₂ from the gas streamwith a scrubbing solution comprising an alkali hydroxide in an amountsufficient to maintain a scrubbing solution pH greater than 5, and c.removing at least a portion of any alkali aerosols generated from thescrubbing step, and oxidized mercury not captured in the scrubbing step,from the gas stream with an aerosol removal means.
 6. The process ofclaim 6, wherein said alkene is at least one taken from the groupconsisting of ethene and propene.
 7. The process of claim 6, whereinsaid electrical discharge reactor is a dielectric barrier dischargereactor.